The invention relates to a method for operating a blast furnace, in particular a cupola furnace, to which metallic charge materials, alloy elements and energycarriers, such as coke, are fed in the upper part of the shaft, and an oxidizing agent, such as air, is fed in the lower part of the shaft, filler materials, namely alloy elements, metal chips or dusts, being supplied to the melting zone.
The introduction of such filler materials into the melting zone separately from one another is standard in the prior art.
For instance, it is known to inject alloy elements, such as carburizing compounds, silicon carbide or ferrosilicon, into the melting zone of cupola furnaces with the aim of correcting the iron analysis (EP 0 336 121 B1)
In recent times, dusts occurring in the foundry and in the operation of cupola furnaces have been injected in the tuyere zone for the furnace wind of the cupola furnace (EP 0 504 700 A1). Such dusts are, for example, filter dusts from cupola furnace dedusting, used foundry sands which can no longer be reconditioned, dusts from the cleaning department and grinding floor, etc. These dusts contain, inter alia, SiO.sub.2 which can be used as slag-forming constituents for the cupola furnace melting process, as well as combustible organic constituents whose calorific value can be used for the melting process. Some of these dusts are considered to be special waste which is expensive to dispose of.
In the case of injection into the melting zone of the cupola furnace, the calorific value of the combustible constituents of the dusts is used as melting energy and the SiO.sub.2 is used as a slag-forming constituent.
Problematic constituents of the dusts, such as heavy metals, are integrated into the slag, e.g. vitrified, in an environmentally friendly manner.
All these filler materials which are introduced into the melting zone of the cupola furnace have to be injected under different and, in some cases, contradictory conditions. Whereas for the alloy elements a deoxidizing atmosphere is required at the point of injection, for dusts which have a high calorific value, there must be an oxidizing operation. In the case of dusts which have a low calorific value, it is expedient to introduce an energy carrier as well, so that the slagging of the SiO.sub.2 proportion of these dusts does not take place at the cost of the coke rate.
In general, the introduction, in particular of dusts, lowers the melting temperature.
To counteract this incompatibility, it was proposed in EP 0 618 419 A1 to provide a burner with supply of fuel and oxygen, and introduce the dusts into the flame of the said burner. In this case, NO.sub.x may form which is harmful to the environment. Moreover, valuable constituents of the filler materials may oxidize, i.e. burn.